1. Field of the Invention
This invention relates to mechanisms for steering or `pointing` transmitted optical radiation beams and/or the `look` directions from which optical radiation beams are received. It also relates to optical communications systems incorporating such mechanisms, more particularly but not exclusively inter-spacecraft communications systems.
2. Discussion of Prior Art
It has been proposed to communicate between spacecraft using laser radiation beams. Each craft is fitted with a communication terminal comprising a source for generating a collimated laser beam modulated with data to be transmitted, the beam being sent via a point-ahead device, a beam-splitter, a fine pointing mechanism and telescope optics to the other craft. The other craft returns to the first a modulated beam at a slightly different wavelength. This is received via the telescope optics and pointing mechanism of the first craft and split off from the transmission beam path by the beam splitter towards a receiver, i.e. an opto-electrical transducer and data decoder. Thus a full duplex communications link is set up between the two spacecraft. A portion of the received beam may be further split off and fed to a sensor array which senses deviation of the beam pointing direction and controls the pointing mechanism so as to track the other spacecraft. The function of the point-ahead device is to impart a relative angular displacement between the transmitted beam and the look direction in which the received beam is sought, i.e. to allow for the relative movement of the two craft during the time which it takes for the radiation to reach and return from the other craft. As well as the fine pointing mechanism which, the terminal may comprise a coarse printing mechanism is used to determine the general direction of the field of view of the communications terminal, the fine pointing mechanism being operable to track the other spacecraft within that field of view. The fine pointing mechanism may comprise two mirrors which receive the beam in succession and which are able to turn, driven by respective servo-mechanisms, about orthogonal axes.
It is desirable for any one craft to be able to communicate via respective duplex links, with more than one other spacecraft. This can be accomplished by fitting the craft with more than one terminal as described above but great advantage is obtained if a single multiplexing terminal, capable of handling a plurality of duplex beam pairs, could be provided. One proposal for such a multiplex terminal is set out in our U.S. Pat. No. 4,933,928. The several beam pairs handled by this apparatus are at different wavelengths and are multiplexed together by an arrangement of dichroic mirrors. Naturally, the beam pairs may need to be pointed in different directions and this is done, upstream of the multiplexing arrangement, by respective fine pointing mechanisms.